$ D = \left[\begin{array}{rrr}3 & 5 & -1 \\ 1 & 0 & -2\end{array}\right]$ $ w = \left[\begin{array}{r}1 \\ 0 \\ 5\end{array}\right]$ What is $ D w$ ?
Answer: Because $ D$ has dimensions $(2\times3)$ and $ w$ has dimensions $(3\times1)$ , the answer matrix will have dimensions $(2\times1)$ $ D w = \left[\begin{array}{rrr}{3} & {5} & {-1} \\ {1} & {0} & {-2}\end{array}\right] \left[\begin{array}{r}{1} \\ {0} \\ {5}\end{array}\right] = \left[\begin{array}{r}? \\ ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ w$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ w$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ w$ , and so on. Add the products together. $ \left[\begin{array}{r}{3}\cdot{1}+{5}\cdot{0}+{-1}\cdot{5} \\ ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ w$ and add the products together. $ \left[\begin{array}{r}{3}\cdot{1}+{5}\cdot{0}+{-1}\cdot{5} \\ {1}\cdot{1}+{0}\cdot{0}+{-2}\cdot{5}\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{r}{3}\cdot{1}+{5}\cdot{0}+{-1}\cdot{5} \\ {1}\cdot{1}+{0}\cdot{0}+{-2}\cdot{5}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{r}-2 \\ -9\end{array}\right] $